Microelectronic imagers are used in digital cameras, wireless devices with picture capabilities, and many other applications. Cell phones and Personal Digital Assistants (PDAs), for example, are incorporating microelectronic imagers for capturing and sending pictures. The growth rate of microelectronic imagers has been steadily increasing as they become smaller and produce better images with higher pixel counts.
Microelectronic imagers include image sensors that use Charged Coupled Device (CCD) systems, Complementary Metal-Oxide Semiconductor (CMOS) systems, or other solid state systems. CCD image sensors have been widely used in digital cameras and other applications. CMOS image sensors are also quickly becoming very popular because they are expected to have low production costs, high yields, and small sizes. CMOS image sensors can provide these advantages because they are manufactured using technology and equipment developed for fabricating semiconductor devices. CMOS image sensors, as well as CCD image sensors, are accordingly “packaged” to protect the delicate components and to provide external electrical contacts.
FIG. 1 is a schematic view of a conventional microelectronic imager 1 with a conventional package. The imager 1 includes a die 10, an interposer substrate 20 attached to the die 10, and a spacer 30 attached to the interposer substrate 20. The spacer 30 surrounds the periphery of the die 10 and has an opening 32. The imager 1 also includes a transparent cover 40 over the die 10.
The die 10 includes an image sensor 12 and a plurality of bond-pads 14 electrically coupled to the image sensor 12. The interposer substrate 20 is typically a dielectric fixture having a plurality of bond-pads 22, a plurality of ball-pads 24, and traces 26 electrically coupling bond-pads 22 to corresponding ball-pads 24. The ball-pads 24 are arranged in an array for surface mounting the imager 1 to a board or module of another device. The bond-pads 14 on the die 10 are electrically coupled to the bond-pads 22 on the interposer substrate 20 by wire-bonds 28 to provide electrical pathways between the bond-pads 14 and the ball-pads 24.
The imager 1 shown in FIG. 1 also has an optics unit including a support 50 attached to the spacer 30 and a barrel 60 adjustably attached to the support 50. The support 50 can include internal threads 52, and the barrel 60 can include external threads 62 engaged with the threads 52. The optics unit also includes a lens 70 carried by the barrel 60.
One problem with packaging conventional microelectronic imagers is that it is difficult to accurately align the lens with the image sensor. Referring to FIG. 1, the centerline of the lens 70 should be aligned with the centerline of the image sensor 12 within very tight tolerances. For example, in microelectronic imagers that have higher pixel counts and smaller sizes, the centerline of the lens 70 is often required to be within a few microns of the centerline of the image sensor 12. This is difficult to achieve with conventional imagers because the support 50 may not be positioned accurately on the spacer 30. Moreover, because the barrel 60 is threaded onto the support 50, the necessary clearance between the threads can cause misalignment between the axes of the support 50 and the barrel 60. Loss in concentricity because of non-coincident axes negatively affects the focus and/or clarity of the imager. Therefore, there is a need to align lenses with image sensors with greater precision in more sophisticated generations of microelectronic imagers.
Another problem of packaging conventional microelectronic imagers is that positioning the lens at a desired focus distance from the image sensor is time consuming and may be inaccurate. The lens 70 shown in FIG. 1 is spaced apart from the image sensor 12 at a desired distance by rotating the barrel 60 (arrow R) to adjust the elevation (arrow E) of the lens 70 relative to the image sensor 12. In practice, an operator manually rotates the barrel 60 by hand while watching an output of the imager 1 on a display until the picture is focused based on the operator's subjective evaluation. The operator then adheres the barrel 60 to the support 50 to secure the lens 70 in a position where it is spaced apart from the image sensor 12 by a suitable focus distance. This process is problematic because it is exceptionally time consuming and subject to operator errors.
Still another concern of conventional microelectronic imagers is the manufacturing costs for packaging the dies. The imager 1 shown in FIG. 1 is relatively expensive because manually adjusting the lens 70 relative to the image sensor 12 is very inefficient and subject to error. The conventional imager 1 shown in FIG. 1 is also expensive because each cover 40 is individually attached to the spacer 30, and each spacer 30 is individually attached to an interposer 20. Moreover, the support 50 and barrel 60 are individually assembled separately for each die 10 after the dies have been singulated from a wafer and attached to the interposer 20. Accordingly, there is a significant need to enhance the efficiency, reliability, and precision of packaging microelectronic imagers.
One aspect of forming the imager 1 is attaching the cover 40 to the spacer 30. The cover 40 can prevent contaminants from impairing the performance of the imager 1. However, one problem with positioning the cover 40 over the image sensor 12 is that the cover 40 can have defects and imperfections that degrade image quality. Furthermore, the defects and/or imperfections on the cover 40 can result in the image sensor 12 malfunctioning and/or becoming inoperable.
In certain cases, the cover 40 can be coated with an anti-reflective (AR) coating and/or an infrared (IR) blocking film to help improve the performance of the imager 1. This process, however, is undesirable because evaporative processes are typically used to deposit the AR coatings and the IR films. Evaporative processes are subject to splattering and/or flaking. For example, evaporative processes typically operate by applying a number of discrete sublayers to achieve the desired optical properties. One problem with this process is that a contaminant (e.g., a particle) can become lodged on an underlying sublayer of the AR/IR coatings and subsequent layers deposited over the underlying sublayer magnify the problem of the particle. This can cause shadowing on the image sensor 12. Furthermore, because the particle is embedded in the film stack, it cannot be removed from the coating. Therefore, there is a significant need to eliminate the performance degradation caused by the glass cover and improve the methods for forming AR/IR coatings.